1. Field of the Invention
This invention relates generally to automatic rear-wheel steering control for a vehicle/trailer combination and, more particularly, to automatic rear-wheel steering control for a vehicle/trailer combination that employs both feed-forward (open-loop) control and feedback (closed-loop) control, where the feedback control employs vehicle yaw rate, trailer yaw rate and vehicle lateral acceleration feedback to improve both vehicle and trailer stability and handling performance.
2. Discussion of the Related Art
It is known in the art to employ automatic rear-wheel vehicle steering based on vehicle dynamic information during a vehicle turn, or yaw. Active rear-wheel steering control can improve vehicle stability over a conventional vehicle having only two steerable front wheels. The rear-wheel steering control can be in-phase steering or out-of-phase steering. In-phase rear-wheel steering steers the rear wheels in the same direction as the front wheels, and is typically provided at higher vehicle speeds. Out-of-phase rear-wheel steering steers the rear wheels in an opposite direction as the front wheels to provide a tighter turning radius, and is typically provided at lower vehicle speeds.
Open-loop rear-wheel steering provides a certain amount of rear-wheel steering control depending on the amount of hand-wheel steering provided by the vehicle operator and the vehicle speed. It is known to provide closed-loop rear-wheel steering based on certain feedback signals in the event that the vehicle is not following the steering path requested by the vehicle operator. Closed-loop rear-wheel steering assist systems sense the actual vehicle yaw rate and the intended yaw rate, and generate an error signal that provides the steering control by the rear wheels if the vehicle yaw rate and the intended yaw rate are not the same.
Directional dynamics and stability have been a primary concern for vehicle/trailer combinations because of certain undesirable response properties for heavy trailers when traveling at high speeds and/or on low coefficient friction surfaces. The handling performance of a vehicle towing a trailer can also be deteriorated as a result of the adverse influence of the trailer dynamics and kinematics. Because the vehicle/trailer combination is a two pivot-connected unit, the trailer is more prone to instability, such as lateral swing or even a jackknife condition. The vehicle is more related to the handling performance because the vehicle operator typically perceives only the vehicle's dynamics.
Compared to the single-unit vehicle, the vehicle operator of a vehicle/trailer combination has an additional task of coping with trailer oscillation, possible instability and the path followed by the trailer. Sometimes, these conditions conflict with each other in control design, particularly for vehicles including automatic rear-wheel steering control. For example, control systems for vehicles including automatic rear-wheel steering control typically attempt to keep the vehicle side-slip zero at the vehicle's center of gravity. However, the center of gravity of the vehicle/trailer combination is different than the center of gravity of the vehicle. The balance between overall system stability and vehicle handling performance often becomes a compromise due to limited control channel availability. In addition, the vehicle/trailer combination has a higher order, and thus is more complex, which further makes the control design more complicated.